Engine performance

[Bruce]

It often gets over looked and forgotten.

It's very easy to perform and check, for most engines. It's one thing I stress to owners to check every 500 hours or one year.

It's part of a geometry (here is where your grade school math teacher was correct) that keeps your engine running smooth and at top performance.

What the hell am I talking about? Valve lash, which is the clearance between the rocker arm/cam lobe and the valve stem.

Some basics.

Non overhead cam engines:
You have a cam shaft that is turned by the crankshaft. This cam shaft has lobes that are eliptical which raise and lower a lifter which moves a push rod up and down which engages a rocker arm on one end and the other end of the rocker arm contacts the valve stem and pushes it down.

The valve spring retains the valve from falling into the cylinder (where the phrase dropped a valve come from) and aides in closing the valve when the rocker arm pressure is reduced for closing.

There are different types of lifters used. Solid, which gave a vlave train its clackity noise, hydraulic, which are buffered by the oil pressure system.
These lifters can have a flat bottom or a roller.

Solid lifters are just that, solid. No give between the cam lobe and push rod. Valve lash is extremely important in this design and are adjusted cold.

Hydraulic lifters use pressure from the oil system to allow them to partialy collapse and buffer the valve train making it quieter. These are almost always adjusted with a hot engine running.

Over head cam engines use two basic designs:
One where the camshaft lobe contacts the valve stem directly. There is usually no adjustment here and when a part wears you replace it.

The other has the cam lobe contact a valve bridge which inturns moves the valve stem. This can have mutliple adjustments and be very difficult to make sure everything is set correctly and takes special tools to perform. I.E. the Yamar 6LP.

Why is it important?

Well as the saying goes, timing is everything. And the engine timing is no exception.

To keep an engine running smooth and at its top performance it needs to deliver its fuel charge and in gas its spark and diesel its compression at the precise time to fire and complete its 3rd cycle of a 4 cycle engine.
Intake, compression, power and exhaust.

All these operations work thru the intake and exhaust valves.

Excess or to little clearance throws the timing off. When the timing is off the engine does not run at its peak performance.

Excess clearance can also lead to pounding of the valve stem and in some case mushroom the head.

Too little may not allow the valve to seat properly in its seat leading to burning of the valve face and seat and loss of compression and back firing thru the intake. in diesel where compression is everything it can lead to poor starting or a no start condition.

Checking it can let you discover wear in many valve train components and repair before major damage.

A good set a feeler stock and some patience can help you keep your engine running smooth. And as I said before, removing the rocker covers and doing a visual can help with potential problems.

NOTE:
On gas engines with hydraulic lifters, adjustment can be somewhat a hassle. Being that you adjust with a hot running engine, oil can squirt everywhere and is a potential fire hazzard. They sell clips that go over the hole in the rocker arm that diverts the lube oil.

But in some cases this is still not enough. Those engines with low wall height where the rocker contacts the head can let the oil leak over the side before it drains back into the engine.

This is where a second set of rocker covers with the top cut off will allow you to do the adjustment trouble free.

A little work and expense, yes but what isn't that's good for your power plant.

[Capt Dick Dean]

My head hurts.

[Eddy G]

Bruce: On a solid lifter cam in block engine such as my 4LH's, does the cylinder that you are adjusting have to be at exactly TDC? It would seem that once the valves are closed and the crank rotated another 100 or so degrees that the valves are on the low side of the cam lobe. Does the gap change from there to TDC?

Eddy G.

[Bruce]

Yes its needs to be on TDC.

Your dealing in thousands of an inch and any variation off TDC starts getting into lobe lift.